1. Field of the Invention
The present invention relates to an objective lens used in an optical pickup apparatus performing an operation for reading signals recorded in an optical disc and an operation for recording signals into an optical disc, an objective lens manufacturing method, and an optical pickup apparatus.
2. Description of the Related Art
An optical disc apparatus is widely used which applies a laser beam emitted from an optical pickup apparatus to a signal recording layer of an optical disc to perform a signal reading operation and a signal recording operation.
While an optical disc apparatus using an optical disc called CD or DVD is generally used, there is recently developed an optical disc apparatus using an optical disc with improved record density, i.e., the optical disc of the Blu-ray standard or HD DVD standard (High Definition Digital Versatile Disc).
Infrared light with a wavelength of 780 nm is used as the laser beam for the operation of reading signals recorded in a CD-standard optical disc, and red light with a wavelength of 650 nm is used as the laser beam for the operation of reading signals recorded in a DVD-standard optical disc.
A protective layer provided on a top surface of the signal recording layer of the CD-standard optical disc has a thickness of 1.2 mm, and a numerical aperture of an objective lens used for the signal reading operation from the signal recording layer is set to a specified value of 0.45. The protective layer provided on the top surface of the signal recording layer of the DVD-standard optical disc has a thickness of 0.6 mm, and the numerical aperture of the objective lens used for the signal reading operation from the signal recording layer is set to the specified value of 0.6.
There is used the laser beam with a shorter wavelength, for example, blue-violet light with a wavelength of 405 nm as the laser beam for the operation of reading signals recorded in the Blu-ray and HD-DVD-standard optical discs, as compared to the CD-standard and DVD-standard optical discs.
The protective layer provided on the top surface of the signal recording layer of the Blu-ray standard optical disc has a thickness of 0.1 mm, and the numerical aperture of the objective lens used for the signal reading operation from the signal recording layer is set to the specified value of 0.85.
It is required to reduce a diameter of a laser spot generated by focusing a laser beam for reproduction of signals recorded in the signal recording layer of the Blu-ray standard optical disc and for recording signals in the signal recording layer. The objective lens used for obtaining a desired laser spot shape has not only an increased numerical aperture (NA) but also a reduced focal distance, and therefore, a curvature radius of the objective lens is reduced.
An optical pickup apparatus includes a laser diode emitting the laser beam with a wavelength corresponding to the above standards and an objective lens focusing the laser beam emitted from the laser diode onto the signal recording layer of the optical discs. Although glass is generally used as a material of the objective lens, synthetic resin has recently been often used so as to produce inexpensively (see Japanese Patent Application Laid-Open Publication No. 2005-338684).
Although a price of an optical pickup apparatus can be reduced by using the objective lens made of synthetic resin, synthetic resin has a distinctive feature that characteristics are changed corresponding to a wavelength of the laser beam to be used. When an objective lens for focusing the laser beam corresponding to the Blu-ray standard and the HD-DVD standard, i.e., a blue-violet laser beam with a wavelength of 405 nm is made of synthetic resin, which is used for the objective lens for focusing the laser beam corresponding to the DVD standard, i.e., a red laser beam with a wavelength of 650 nm, for example, synthetic resin called APEL 5014DP of Mitsui Chemicals, inc., transmittance of the objective lens is reduced by the blue-violet laser beam.
FIG. 4 is a characteristic view showing change in transmittance with time in the case of using the objective lens, as it is, which is made of the above synthetic resin called APEL 5014DP to apply the blue-violet laser beam with a wavelength of 405 nm. It is experimentally confirmed that a change in the transmittance of the objective lens with time according to the application time is greater than such specifications applicable to the objective lens as to satisfy optical performance of the optical pickup apparatus. That is, it is confirmed that the above change exceeds the predetermined range of 2%, which is set for the case of 1000-hour application of the blue-violet laser beam having a wavelength of 405 nm, so that the change goes out of the predetermined range to reduce the transmittance.
As the transmittance of the objective lens is reduced as above, intensity of the laser beam focused on the surface of the signal recording layer of the optical disc is reduced, thus reproducing and recording operations cannot normally be performed for signals recorded in the optical disc. Although synthetic resin, which is unaffected by the blue-violet laser beam, for example, synthetic resin called ZEONEX 340R of Zeon Corporation, is used as a material of the objective lens to solve such problem, this synthetic resin is expensive as compared to the material used for the objective lens for DVD, which cause a problem that the price of the optical pickup apparatus cannot be reduced.